Magnetic-core induction furnace



y 1930- P. E. BUNET 1,760,741

MAGNETIC CORE INDUCTION FURNACE Filed May 25, 1928 2 Sheets-Sheet l May 27, 1930. P. E. BUNET I MAGNETIC CORE INDUCTION FURNACE 2 Sheets-Sheet 2 Filed May 25. 1928 Emit] Patented May 27, 1930 UNITED STATES PATENT OFFICE.

PAUL'EFIMANUEL BUNET, OF VERSAILLES, FRANCE, ASSIGNOR TO SOCIETE ACIERIES DE GENNEVILLIERS, OF GENNEVILLIERS, SEINE, FRANCE, A CORPORATION OF FRANCE MAGNETIC-CORE INDUCTION FURNACE Application filed May 25, 1928, Serial No. 280,584, and in France June 2, 1927.

Induction furnaces vemployed at ordinary frequencies and at low frequencies, sometimes also utilized at high frequencies, comprise generally an iron core aroundwhich are a primary circuit and a secondary circuit often in the form of a gutter.

Although the inductor circuit can be applied fairly closely upon the iron core, the same cannot be done as regards the secondary for numerous reasons; the induced or secondary circuit must have a large volume corresponding to the amount of material it is desired to treat at one time; this secondary circuit must be removed from delicate parts because 1t is hot and because certain operations have to be carried out upon this material under treatment.

Under this form, an induction furnace has necessarily a low power factor, by reason of the high magnetic leakages between the two circuits. The present invention remedies this disadvantage to a considerable extent.

It consists in the fact that the furnace being fed by means of a very low tension, almost equal to the tension necessary in the secondary circuit or to a low multiple thereof, the inductor circuit of the furnace consisting of a verysmall number of turns, is applied as closely as possible to the secondary circuit, whatever be its shape and contours. The low tension of supply involves a primary transformation from the tension of the network, but this necessity is largely compensated by the improvement of the power factor and the advantages realized.

Upon the accompanying drawings Figure 1 represents in axial section a diagram of the arrangement habitually em ployed. y

* Figure 2 is an analogous diagrammatic view of an arrangement according to the invention.

'mhat' w e mg Figure 6 is an axial section of the device applied to the heating of another kind of crucible.

Figure 7 is a section along the line 77 of Figure 6.

Figure 8 is an axial section of another application.

Figure 9 is a horizontal section along the line 99 of Figure 8.

Figures 10, 11 and 12 represent modifications of the inductor coil. 7

Figure 13 is an axial section of a modification of the arrangement represented in Figure 8. I 1

Y Figure 14 gives a diagram of connections of the alternator feeding the circuit.

Figure 1 of the annexed drawing represents the diagram of the customary arrangement with inductor circuit P arranged uponthe iron core N.

Figure 2 represents a diagram of an arrangement according to the invention, where the inductor circuit P consisting of a single turn, or a very small number of turns, is applied over the secondary circuit S. The P circuit being of very low tension may remain bare, having no other insulation than the materials of the furnace themselves such as brick, or substances of high mechanical Figure l, the known method, to the novel form shown in Figure 2.

But a still further gain can be obtained,

reducing by more than half the reactive power absorbed, by sub-dividing the circuit P in accordance with the arrangement represented in Figure 3. This circuit P will then'be split into two parts, one in the interior of S, the other external, each of them comprising a small number of turns or a single turn. These two parts are connected together in series or in parallel.

The applicant has ascertained that a furnace of this kind, despite the necessity of a primary transformation from the supply voltage, while increasing the working facilities of the chemist or met-allurgist, lead to a power factor very much higher overall than the arrangement of Figure 1.

The two successive transformers may have certain parts .of their magnetic circuit in commen, with the object of economizin material.

- circuit of very few coils and can be divided into two parts one internal and the other external of the winding R in orderfurther toincrease the power factor. By means of thick bars K, forming a loop enclosing a very small area, the circuit P feeds a circuit P smgle or double as indicated in Figures 2 and 3 respectively. The connections between these circuits P and P each of which may be either single or multiple, can be carried out in any known manner.

It is to be noted that the novel arrangement ust described difi'ers essentially from the old idea, which moreover has not attained any great success because it is insufiicient, known sometimes as the Gronwall type or the shielded core type, in which it was indicated simply that auxiliary circuitshaving at first glance some resemblance with P and F were applied to the core as much as possible in order to reduce the magnetic leakages from the core, departing for this purpose altogether from the contour of the circuit S circular or not. In the present instance, on the contrary, this circuit is followed absolutely as closely as possible and whatever be its shape, preferably upon its two faces, departing for that purpose as far from the core as is necessary. The underlying idea, liketheresults attained, is thus quite difierent.

Again, in order to obtain a slight advantage, circuits P may be arranged also upon the bottom of the coil S as represented at Q; in "Figured, and even upon the top of the coil S thus closing it in on all sidesf But it is necessary to reckonwith the facilities of the operation to be carried out which often requires an inspection of the whole upper surface of the coil S. In the case of three parts P P Q, these can be unitedi'n one coil of U-shape around S or in two parts in the shape of reversed angles L.

The same improvements are applicable to any other type of induction furnace, for example to a furnace in which the heated circuit constitutes only a portion of the total volume of the material under treatment, as represented diagrammatically by Figure 5. The circuit S is of reduced volume; the conductive material under treatment becomes heated therein and receives at the same time a to and fro movement towards and back from a crucible C of greater volume where the operation is carried out, by means of a certain number of two-way channels D, this movement being determined in any known manner by forces due for example to differences of density or to electromagnetic forces. The simplicity of the circuits P and their robustness is favourable to the obtaining of these latter.

Similarly, by way of a further example, there is represented inFigures 6 and 7, a large crucible G with a hole T into which 15 threaded the core N of a magnetic circuit. Ordinarily the primary circuit connected to the network, is a delicate fragile winding, of

medium or high tension, and is placed upon the portion N in the hole T. There is seen immediately the whole advantage of placing in this hot and badly protected position only massive circuits of the kind in question, while reserving the core N for the winding connected to the network and for a circuit P connected to 1 as in the arrangements of Fi ures 4 and 5.

igure 8 represents another example of a furnace having a crucible C around which is placed, without any special electrical nsulation, an inductor circuit P consisting of heavy conductors fed at very low tension by means of the circuit P situated in proximity to the primary R; P and B being arranged around the magnetic coil N. The whole can be stationary or tilting as one unit as desired. Also, this being extremely important for the suppression of explosions in case of perforationv of the crucible, the circulation of water in the lower portion of the inductor circuit P can be suppressed and this water retained only in a kind of gutter 2) arranged at the top of this turn (see Figure 10), the heat reaching the Water by the thermal conductivity of the copper plate. A similar gutter may be arranged at the top of Sometimes also the water can be completely suppressed. i i

Recourse may further be had to the cooling of-the circuit P by means of air forced in by a fan. Whereas this arrangement is diflicult to adopt in the case of ahelix with numerous coils by reason of the loss of head brought about by th'elength and smallQcross-section of the passage, it is easy to employ in this case a suitable arrangement of which Figure 11 is an example. The addition of the wall p all round P or only over a portion of its periphery, will allow the supply of air of which the cooling effect will be increased if necessary by enlarging the surface of P by means of projections like in any radiator. The cooling efi'ect of the air may be largely increased by an atomization of water. The wall 10 may participate or not in the circulation of the electric current.

Instead of a simple conductor P there may be arranged around the crucible a number of big coils (two or three) arranged in series, or any number in parallel, fed at fairly low tension in order to need only the insulation constituted by the refractory materials surrounding the crucible, without danger of shocks for the workmen. In the case of two or three conductors arranged in this way, coolingvcan be ensured as in the previous instance by air or by water circulating at the top.

In the case of juxtaposed conductors in parallel, as for example in Figure 12, it may sometimes be advantageous to separate them over a large portion of the height by a thin insulator P asbestos for example; in this way, there will be obtained a reduction in the resistance arising fromthe Kelvin effect.

Lastly, the magnetic core N may be extended above the crucible, and pass around, or again one or more special cores may be added at these positions, in order partially to conduct the flux passing into the crucible, according to the specifications of the application for patent filed by me on the same day for Improvements in high frequency induction furnaces, Serial Number 58,083.

Figure 13 represents a modification of Figure 8. The circuit P is divided in two, one portion being inside N, the other outside; the two portions P P are connected in series or in parallel and connected always directly to the turn P In this way the conductors of the circuits P and R are utilized to the best advantage, with a material reduction, as the frequency rises, of the effective resistance due to the Kelvin or skin eflect and the analogous effects due to the proximity of the circuits.

The primary winding R (Figure 18) may comprise various current tappings, either for adjustment, or for allowing the construction of the alternator, the con enser and the furnace at the most suitable tension for each of them, the feed being effected for-example as shown in Figure 14.

It will be understood that the arrangements which precede may be applied to induction furnaces of any frequency for the use of iron constitutes an advantage.

. Instead of single-phase furnaces, there may also be employed polyphase furnaces and the same principle applied to them, these polyphase' furnaces comprising several magnetic cores, several circuits such as R P P and several heating circuits S of any known kinds these difl'erent circuits S being separate or connected, or extending to a single general crucible analogous to C of Figure 5.

What I claim is:

1. In a magnetic core induction furnace, comprising a primary heating circuit, consisting of a very small number of turns arranged as close as possible to the secondary heated circuit, and transformer means for feeding the said primary circuit with current of very low tension, the provision of the said primary and secondary circuits around one of the branches of a single closed magnetic circuit, the other branch of which forms the core of the said transformer means.

2. In a magnetic core induction furnace, comprising a primary heating circuit, consisting of two turns arranged, the one externally and the-other internally of the secondary heated circuit, as close as possible to the latter, and transformer means for feeding the said primary circuit with current of very low tension, the provision of the said primary and secondary circuits around one of the branches of a single closed magnetic circuit, the other branch of which forms the core of the said transformer means.

3. In a magnetic core induction furnace, comprising a primary heating circuit, consisting of a very small number of turns, cooled at their upper part only, arranged as close as possible to the secondary heated circuit; and transformer means for feeding the said primary circuit with current of very low tension, the provision of the said primary and secondary circuits around one of the branches ofa single closed magnetic circuit, the other branch of which forms the core of the said transformer means.

In testimony whereof I have signed my name to this specification.

PAUL EMMANUEL BUNET. 

